DFS Used space is not correct computed on frequent append operations

Details

Description

The current implementation of the Datanode will increase the DFS used space on each block write operation. This is correct in most scenario (create new file), but sometimes it will behave in-correct(append small data to a large block).
For example, I have a file with only one block(say, 60M). Then I try to append to it very frequently but each time I append only 10 bytes;
Then on each append, dfs used will be increased with the length of the block(60M), not teh actual data length(10bytes).
Consider in a scenario I use many clients to append concurrently to a large number of files (1000+), assume the block size is 32M (half of the default value), then the dfs used will be increased 1000*32M = 32G on each append to the files; but actually I only write 10K bytes; this will cause the datanode to report in-sufficient disk space on data write.